(i) Field of the Invention
The present invention relates to resist film coating apparatus and more particularly to a resist film coating apparatus which is improved to permit formation of a uniform resist film on a wafer used for an integrated circuit and the like.
(ii) Prior Art
Recently, with high degree of integration of integrated circuits encouraged, fine formation of a pattern formed on a wafer has further been promoted and high dimensional accuracies have been required. Accordingly, the role of lithography techniques considered to be in the heart of the fine formation of patterns has added to its importance more and more. Namely requirements imposed on exposure techniques and resist formation process as well as apparatus used therefor have become more stringent and have been diversified.
A conventional resist film coating apparatus used for formation process of a resist film on a wafer employs a spin coat method in which resist liquid is dropped on the central part of the wafer and the wafer is rotated at a constant revolution number to shake off the resist dropped on the wafer, thus forming a resist film of a desired thickness.
In the conventionally available spin coat method for resist film formation, however, the thickness of a resist film to be coated is not monitored constantly but is monitored at intervals during spin coating. This leads to a disadvantage that the resist film thickness sometimes deviates from an intended value to prevent formation of a pattern of an intended line width.
In connection with measurement of the thickness of resist films, on the other hand, an invention concerning a similar technique to the resist film coating apparatus and entitled "Substrate Processing Apparatus" is disclosed in Japanese Patent Application Laid-open No. Hei 1-276722, laid open on Nov. 7, 1989.
Described therein is a substrate processing apparatus in which a resist film is coated on a wafer by using the aforementioned spin coat method and a resist pattern obtained by selectively exposing the resist film is developed.
More particularly, the described substrate processing apparatus is as shown in FIG. 1 and it comprises a spin drive 2 for fixedly carrying a wafer 1 formed at its surface with a film and rotating the film at a high speed, a cup 3 for covering the spin drive 2 and wafer 1 and a liquid chemical nozzle 5 reciprocative radially of the wafer 1. This Japanese Patent Application Laid-open No. Hei 1-276722 also describes that the substrate processing apparatus further comprises a position detector 8 for measuring the distance of the liquid chemical nozzle 5 in motion from the center of the wafer 1, a plurality of film thickness measuring elements 9, 10 and 11 arranged above the wafer 1 radially thereof and an operation/control unit 12 operative to control the discharge amount of liquid chemicals, the reciprocating speed of the liquid chemical nozzle 5 and the revolution number of the wafer on the basis of measurement results delivered out of the film thickness measuring elements 9, 10 and 11, whereby the progress state of development affected by irregularities in the thickness of a coated film on the wafer 1 and in the exposure amount is monitored by the plurality of film thickness measuring elements 9, 10 and 11 arranged radially, differences in developing rate over the wafer surface are measured by means of the plurality of film thickness measuring elements, and results of measurement are fed back to control the discharge amount of liquid chemicals, the reciprocating speed of the liquid chemical nozzle and the revolution number of the wafer, thereby ensuring that defective due to irregularities in pattern dimension over the entire surface of the wafer can be eliminated.
However, the invention described in the aforementioned Japanese Patent Application Laid-open No. Hei 1-276722 is directed to measurement of a film thickness on an actual wafer and has difficulties in being applied to all the processes of production of an actual device. For example, when a substrate having a silicon oxide film and a silicon nitride film which underlie a resist is measured with a film thickness meter of interference type, thicknesses of the underlying transparent films are inevitably included in the results of measurement.